Invert weight paint values with modifier
Yes. (Besides being able to invert the influence of the vertex group in many of the modifiers, by hitting the small double-arrow icon next to the Vertex Group field....) 

If you want to invert the group itself, you can use the Vertex Weight Mix modifier targeted on the group (here, 'Group'), and mix with an anonymous Group B, with B's default weight set to 1, and the Mix Mode set to 'Difference'.



(Excuse the grotty color quantization)